Apparatus and techniques for direct sequence, spread spectrum modulating and demodulating data signals are well known. Modulation involves generating a periodic comparatively high frequency, repetitive pseudo noise code (PN code) and effectively mixing the data signal with a PN code as with an exclusive OR gate or a balanced mixer. The resulting signal is characterized by a very wide bandwidth and very low spectral energy density. To decode or demodulate a received spread spectrum signal, it is necessary to generate a decoding signal corresponding to the particular PN code previously used for encoding purposes and to apply both the received signal and the PN code to a balanced mixer or other means commonly referred to as a correlator.
It is known that spread spectrum modulation and demodulation are particularly useful for the transmission of data signals such as analog voice signals or digitized data in building interiors over radio frequency (RF) carriers. The low spectral density characteristic of such signals reduces the tendency for interference with other radio sensitive equipment. Additionally, spread spectrum techniques are known intrinsically to reduce interference between multiple reflected versions of a transmitted signal, as minimal phase differences between the local PN decoding signal and reflected signals results in low signal correlation and consequently demodulation of such multiple signals.
Acquisition and code tracking of Direct Sequence Spread Spectrum has traditionally been performed by one of two methods: (1) serial acquisition or (2) matched filter/full parallel acquisition. Serial acquisition has traditionally been used for continuous transmission systems where long acquisition times do not impair system performance. Matched filter/full parallel acquisitions have been generally applied to burst transmissions such as those used in radio packet data networks and wireless LANs where PN code lengths and data spreading ratios are greater than 100 chips. The disadvantage of the parallel approach is the excessive hardware complexity for a digital implementation and the high insertion loss and inflexibility to change data bandwidths and chip and signalling rates for SAW (Surface Acoustic Wave) matched filter implementations.
In general, prior art structures and techniques are not conducive to implementation in portable battery operated commuting and communications systems. Wireless packet networks and LANs use short PN codes. Accordingly, it appears that a hybrid multiple serial correlator approach may be used to increase acquisition rates, and reduce hardware complexity while providing complete flexibility to perform symbol, chip rate and code length changes within a packet burst.
In the prior art, as in U.S. Pat. No. 4,774,715, a basic multiple correlator acquisition system based on analog correlators is shown, but the same fails to provide the desired characteristics of optimal size and power reduction necessary for use in a portable battery operated system.